[STJ] Replace module-wide SkipLocalsInit with per-method [SkipLocalsInit]

[EgorBo]

experiment to see impact on asmdiffs

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This PR description was authored by GitHub Copilot.

[EgorBo]

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[Copilot]

Pull request overview

This PR changes System.Text.Json’s handling of SkipLocalsInit from a project/module-wide setting to targeted per-method [SkipLocalsInit] attributes, primarily on hot paths that use stackalloc. It also adds a downlevel polyfill for SkipLocalsInitAttribute so the attribute can be used when targeting frameworks that don’t provide it.

Changes:

• Set <SkipLocalsInit>false</SkipLocalsInit> in System.Text.Json.csproj and apply [SkipLocalsInit] to specific methods across writer/reader/DOM/converters.
• Add an internal polyfill System.Runtime.CompilerServices.SkipLocalsInitAttribute for non-.NETCoreApp TFMs.
• Add using System.Runtime.CompilerServices; where needed.

Reviewed changes

Copilot reviewed 48 out of 48 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/Common/src/Polyfills/SkipLocalsInitAttributePolyfill.cs	Adds downlevel SkipLocalsInitAttribute polyfill for non-.NETCoreApp targets.
src/libraries/System.Text.Json/src/System.Text.Json.csproj	Disables project-wide SkipLocalsInit and wires in the polyfill for non-.NETCoreApp TFMs.
src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.TryGetProperty.cs	Applies [SkipLocalsInit] to stackalloc-heavy property lookup paths.
src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs	Applies [SkipLocalsInit] to stackalloc-heavy string comparison helper.
src/libraries/System.Text.Json/src/System/Text/Json/JsonEncodedText.cs	Applies [SkipLocalsInit] to transcoding/encoding helper using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/JsonHelpers.Escaping.cs	Applies [SkipLocalsInit] to escaping helpers using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/JsonHelpers.cs	Applies [SkipLocalsInit] to helper using stackalloc buffers.
src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonArray.cs	Applies [SkipLocalsInit] to JSON-path building logic using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonNode.cs	Applies [SkipLocalsInit] to GetPath() which uses ValueStringBuilder(stackalloc ...).
src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.Unescaping.cs	Applies [SkipLocalsInit] to unescaping/transcoding helpers using stackalloc and pooling.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.cs	Applies [SkipLocalsInit] to escaped DateTime/Guid parsing helpers using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.MultiSegment.cs	Applies [SkipLocalsInit] to multi-segment literal validation path using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.TryGet.cs	Applies [SkipLocalsInit] to string-copy and DateTime/Guid TryGet paths using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.cs	Applies [SkipLocalsInit] to ValueTextEquals(ReadOnlySpan<char>) which uses stackalloc/pooling.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/CharConverter.cs	Applies [SkipLocalsInit] to stackalloc-based formatting/parsing path.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/DateOnlyConverter.cs	Applies [SkipLocalsInit] to stackalloc-based parsing/formatting helpers.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs	Applies [SkipLocalsInit] to stackalloc/pooling enum formatting/parsing helpers.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/HalfConverter.cs	Applies [SkipLocalsInit] to stackalloc-based floating-point formatting/parsing helpers.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int128Converter.cs	Applies [SkipLocalsInit] to Int128 parsing/formatting helpers (stackalloc/pooling).
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/TimeOnlyConverter.cs	Applies [SkipLocalsInit] to stackalloc-based parsing/formatting helpers.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/TimeSpanConverter.cs	Applies [SkipLocalsInit] to stackalloc-based parsing/formatting helpers.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/UInt128Converter.cs	Applies [SkipLocalsInit] to UInt128 parsing/formatting helpers (stackalloc/pooling).
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/VersionConverter.cs	Applies [SkipLocalsInit] to stackalloc/pooling version parsing helper.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs	Applies [SkipLocalsInit] to span/string reading paths using stackalloc/pooling.
src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs	Applies [SkipLocalsInit] to truncate helper using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/JsonWriterHelper.Date.cs	Applies [SkipLocalsInit] to date formatting helpers using stackalloc.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/JsonWriterHelper.cs	Applies [SkipLocalsInit] to transcoding/escaping helpers using stackalloc/pooling.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.cs	Applies [SkipLocalsInit] to hot writer paths using stackalloc buffers.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Bytes.cs	Applies [SkipLocalsInit] to stackalloc-based property-name writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.DateTime.cs	Applies [SkipLocalsInit] to stackalloc-based DateTime property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.DateTimeOffset.cs	Applies [SkipLocalsInit] to stackalloc-based DateTimeOffset property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Decimal.cs	Applies [SkipLocalsInit] to stackalloc-based decimal property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Double.cs	Applies [SkipLocalsInit] to stackalloc-based double property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Float.cs	Applies [SkipLocalsInit] to stackalloc-based float property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.FormattedNumber.cs	Applies [SkipLocalsInit] to stackalloc-based formatted-number property-name writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Guid.cs	Applies [SkipLocalsInit] to stackalloc-based Guid property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Literal.cs	Applies [SkipLocalsInit] to stackalloc-based literal property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.SignedNumber.cs	Applies [SkipLocalsInit] to stackalloc-based signed-number property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.String.cs	Applies [SkipLocalsInit] to stackalloc-based string property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.UnsignedNumber.cs	Applies [SkipLocalsInit] to stackalloc-based unsigned-number property-name/value escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Decimal.cs	Applies [SkipLocalsInit] to stackalloc-based decimal value writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Double.cs	Applies [SkipLocalsInit] to stackalloc-based double value writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Float.cs	Applies [SkipLocalsInit] to stackalloc-based float value writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Raw.cs	Applies [SkipLocalsInit] to raw-value transcoding path; also includes an incidental header encoding/whitespace change.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.SignedNumber.cs	Applies [SkipLocalsInit] to stackalloc-based signed-number value writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.String.cs	Applies [SkipLocalsInit] to stackalloc-based string escaping/writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.StringSegment.cs	Applies [SkipLocalsInit] to stackalloc-based segmented string/base64 writing.
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.UnsignedNumber.cs	Applies [SkipLocalsInit] to stackalloc-based unsigned-number value writing.

Comments suppressed due to low confidence (2)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int128Converter.cs:67

• If Int128.TryParse fails, this method throws before returning the rented ArrayPool buffer, leaking it. Wrap the parse / throw path in a try/finally (or return the buffer before throwing) so rentedBuffer is always returned, including on FormatException paths.

            int bufferLength = reader.ValueLength;

            byte[]? rentedBuffer = null;
            Span<byte> buffer = bufferLength <= JsonConstants.StackallocByteThreshold
                ? stackalloc byte[JsonConstants.StackallocByteThreshold]
                : (rentedBuffer = ArrayPool<byte>.Shared.Rent(bufferLength));

            int written = reader.CopyValue(buffer);
            if (!Int128.TryParse(buffer.Slice(0, written), CultureInfo.InvariantCulture, out Int128 result))
            {
                ThrowHelper.ThrowFormatException(NumericType.Int128);
            }

            if (rentedBuffer != null)
            {
                ArrayPool<byte>.Shared.Return(rentedBuffer);
            }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/UInt128Converter.cs:67

• If UInt128.TryParse fails, this method throws before returning the rented ArrayPool buffer, leaking it. Ensure rentedBuffer is returned in all paths (e.g., via try/finally) so invalid JSON inputs don't permanently consume pooled buffers.

            int bufferLength = reader.ValueLength;

            byte[]? rentedBuffer = null;
            Span<byte> buffer = bufferLength <= JsonConstants.StackallocByteThreshold
                ? stackalloc byte[JsonConstants.StackallocByteThreshold]
                : (rentedBuffer = ArrayPool<byte>.Shared.Rent(bufferLength));

            int written = reader.CopyValue(buffer);
            if (!UInt128.TryParse(buffer.Slice(0, written), CultureInfo.InvariantCulture, out UInt128 result))
            {
                ThrowHelper.ThrowFormatException(NumericType.UInt128);
            }

            if (rentedBuffer != null)
            {
                ArrayPool<byte>.Shared.Return(rentedBuffer);
            }